1. Field of the Invention
This invention relates to digital frequency discriminator circuits, more particularly, to digital frequency discriminators capable of determining whether the frequency of an input pulse train lies within a predetermined frequency band with upper and lower frequency limits for a predetermined period of time.
2. Description of the Prior Art
Various prior art devices have been developed to detect the presence of an optical input pulse train.
U.S. Pat. No. 3,831,039 (Henschel) includes four separate pulse generator circuits operating in combination with two spaced apart electronic switches to determine whether the frequency of an input train lies above a predetermined frequency limit. The circuit taught by Henschel is configured as a digital high pass filter and can determine only whether the frequency of an input pulse train exceeds a predetermined frequency threshold.
U.S. Pat. Nos. 4,162,477 and 4,230,992 (Munkberg) discloses a remote control system for a traffic signal control. This system includes an optical signal discriminator, the primary elements of which are controlled and synchronized by an eight phase master clock or pulse generator. The presence or absence of an input pulse train has no effect on the clock synchronized circuit which runs continuously. This system is synchronized to the eight phase clock and is not synchronized with respect to the input pulse train. The Munkberg optical signal discriminator is manufactured and sold by 3-M and is housed in a plastic enclosure having two spaced-apart, tubular optical input ports as depicted in a 3-M Opticom Priority Control System brochure.